I just finished a project engine for my Jeep CJ-7. It is a 383 ci chevy with 1991 GM tuned port fuel injection. I designed the engine to perform best in the low to mid RPM range as this is an offroad vehicle. I just finished breaking in the engine and am in the tuning process. This is my first smallblock chevy. I am an automotive hobbyist and an electronic engineer by trade. My current problem is that I am experiencing part throttle "pinging" at slow speeds and heavy loads such as top gear moderate acceleration from say 30 mph or so. Under full throttle the detonation goes away. The detonation occurs with vacuum readings between 10" and 5". I am using a stock GM 7730 ecm and PROM from a 1991 Corvette. My feeling is that there is too much advance "burned in the stock chip" for my particular engine versus what the stock corvette wanted to see. I do run 92 octane fuel. My engine has a static compression ratio of 9.5:1(I measured 175psi cranking pressure) and I have Edelbrock aluminum cylinder heads and my cam intake duration is a modest 204 deg @ 0.050 lift 112 lobe separation installed 5 degrees advanced. I set up the squish height at 0.040" (0.028" head gasket plus 0.012 piston deck height) I did this for the purpose of octane tolerance and the recommendation of Keith Black who makes my hypereutectic pistons(12cc D-dish flat tops). As I understand this, the tight squish area will generate lots of turbulence at TDC and speed up the burn rate. The stock corvette engine called for 6 degrees BTDC base timing and I have retarded my timing back as far as 6 degrees AFTER top dead center and still get some pinging. Some people have told me that a small amount of detonation is ok in this case since the engine is not putting out a lot of power in this operating range. My question is this: Is it possible that my engine could require MUCH less ignition advance than a corvette L98 engine in this part throttle region due to burn rate differences?? Since I have already retarded the timing 12 degrees, it makes me think there is something else going on, like possibly a lean cylinder due to one of the injectors having reduced flow.
Your thoughts would be much appreciated.


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Dave Zelinka

 

Are you using a stock Vette chip? You are aware that with the 730 ecm you have a parameter to control the displacement of the engine? Also what type of injector (flow rate) and psi are your running.

You may be hitting the max duty cycle of your injectors and your ecm definitely would incorrect information to for calculate the air flow.

I would suggest going to the PROM board to learn how to burn your own PROM. A "quick fix" may be to either bump up your fuel pressue and/or go to larger fuel injectors. But you really need a custom PROM for your engine.

Start by reading Traxion's article on Programming your PROM in the Articles Section. Follow EVERY link given in the article. Then go to the PROM board and do a key word search on "start".. there has been a dozen guys asking that question and it will give you all the places to learn.

Now that I am getting into burning my own PROM, I amazed about all the parameter you have control of AND some interesting results in just changing the base timing or T-stat without making a corresponding change to the PROM.

 

Stock chips have spark advance curves designed to compensate for EGR. Those advance profiles are way too fast for an engine such as yours. Also, your engine breathes better than the average stocker which would, again, raise cylinder pressures above what the stock chip would expect.

You'll need an aftermarket chip or burn a custom chip for yourself.

 

Thanks for the response.
Glenn, I am running a stock chip. I upgraded from 22lb to 24 lb injectors based on the percent displacement increase. I have a diacom and my pulse widths are ok. My pressure has been adjusted to 46psi. I am thinking maybe one injector only has low flow and therefore one cylinder is a little lean and it is knocking. I have been considering PROM burning, thanks for the input.
JRR: Thanks for your thoughts, kinda what I was thinking too. I did remove the EGR from my system so maybe I need less advance in this part throttle high load area of the engine. Time for a custom PROM I guess

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Dave Zelinka

 

Definitely, an injector inbalance could cause that. Are you getting these "knocks" at normal part-throttle cruise? Also, what does your Diacom say when you have the EGR blocked off? Does it still indicate the EGR is "working" even though it is not, or do you just stay at 0%?

I am a bit surprised about the low vacuum reading given the cam you have. Have you checked for possible vacuum leaks? That would definitely a source of your problems if you had a minor leak.

 

There are two ways to rate octane. Motor and research. The pump number is an average between the two. One method (I forget which) is more susceptable to detonation at part throttle and the other is a test of full throttle detionation resistance. Some combinations of fuel will have a higher motor or research than others. You might get rid of the rattle by trying a different gas. You might be able to drop down one heat range on the plug also.

[This message has been edited by jcb999 (edited December 19, 2000).]

 

Glenn: Yes I am getting these knocks at normal part throttle. The exact area is 5" to 10" vacuum 1000-2200rpm. Diacom shows computer is commanding up to 10 deg retard at times. I have the EGR turned off along with VATS on my chip. My spark table calls for 30 deg advance(plus 6deg base) at 1200rpm and 70 kPa. This is stock for a 1991 corvette, but the same year camaro calls for 10 deg less advace at the same point. I am gonna try loading the camaro spark table and see what happens. My motor has 0.040" quench height(high turbulence at TDC/faster burn), high cylinder pressure at low rpm(due to early intake valve closing since I have a short duration cam installed with 5 degrees advance) and no EGR are all leading to excessive cylinder pressure. Plus the spark tables are for a 350 ci motor, a 383 will have better volumetric efficiency(more effective compression) at low rpm and hence want less spark advance....... Thats my thinking anyway, thanks for your response!

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Dave Zelinka